Response to CREG announcement "Effects of New Anticancer Drugs on the Cell Cycle" DCT-6 NIH Grants and Contracts, Vol. 5, No. 20, Nov. 30, 1976. The overall objective of this research is to characterize the cell cycle phase-dependent sensitivity of new anticancer drugs both in vitro and in vivo. These studies will employ a new method for selection of synchronized mammalian cells, centrifugal elutriation separation, which has several distinct advantages over conventional synchrony techniques. The ease and speed of the technique and the degree of synchrony obtained makes testing the cell cycle stage sensitivity simple and independent of pertubations caused by induced synchrony techniques. The cell cycle stage sensitivity will be assayed for several different cell lines in culture, including a fibrosarcoma line which also grows in mice as a solid tumor. The same cell line will be assayed for its cell cycle stage sensitivity to drugs in vivo and the results will be compared to the in vitro studies. Ultimately, the results of these investigations may provide new information to improve use of the drugs.